A multi-center study of oxacillin resistant S. aureus is in place and should be completed within a year. Phage types have been, and will continue to be used to fingerprint strains of the oxacillin resistant species. We have evidence that strains with oxacillin resistance not reversible by clavulanic acid are few in number, and that oxacillin resistant S. aureus is represented in many U. S. hospitals by a single predominant strain. S. aureus in which oxacillin resistance is reduced in the presence of clavulanic acid, the hyperbetalactamase producers, have another, distinct phage pattern. We also are studying loss rates of oxacillin resistance after the strains are cultured from the treated patient. This phase will include initial sensitivity tests done directly from clinical samples, followed by tests done during serial subcultures. This has important bearing on sensitivity testing of these S. aureus strains, since they begin to lose resistance immediately, on initial cultures. Progress: This study is completed. It turned out to be very interesting. It supplied confirmatory as well as new data on the epidemiology of oxacillin resistant S. aureus (ORSA) and hyperbetalactamase producing S. aureus (HBLPSA). ORSA and HBLPSA are distinct strains of epidemic S. aureus. ORSA representing the world-wide phage group III epidemic strain are disappearing from the Clinical Center. Two recent ORSA isolates in the Clinical Center, unrelated to the world-wide strains, were community- derived. HBLPSA is comprised of a single phage Group V strain, in the Clinical Center and in many U. S. Hospitals. This S. aureus strain has an elevated capacity to cause infections. The study to determine loss rates of oxacillin resistance, during in vitro growth, was dropped. Some variables were deemed too difficult to control, and technical help was unavailable.